low fuel rail press 559 code

[Shipit]

Isx cm2250 600,000 miles new pump installed before I bought the truck by warranty maybe 150 to 200 thousand miles ago. I operated it for a couple years now. Fault appeared heavy loaded on a hill I did fuel filters. Problem reoccured same hill. Next checked bleed down on rail it was less than 150 lbs in 30 seconds. During test pressure didn't go above 29,700 psi is that kinda low? I removed fuel pump head found excellent looking steel plungers , cam rollers great and was satisfied with appearance of pump.
!!!!! Except one plunger don't spring back during push test????. Quickserve says if this happens then a check valve in the pump head may be bad. OK then what? I guess Monday I will try to get some help at Cummins Cincinnati maybe they can sell me a new pump head. Any pump experts here??? Thank you very very much[/b]